Question
Check whether the following matrices are invertible or not. : $\left[\begin{array}{ll}1 & 1 \\ 1 & 1\end{array}\right]$

Answer

Let $A=\left[\begin{array}{ll}1 & 1 \\ 1 & 1\end{array}\right]$
Then, $|\mathrm{A}|=\left|\begin{array}{ll}1 & 1 \\ 1 & 1\end{array}\right|=1-1=0$.
$\therefore \mathrm{A}$ is a singular matrix.
Hence, $A^{-1}$ does not exist.

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